Semiconductor memory devices are memory devices that are implemented using semiconductors such as silicon (Si), germanium (Ge), gallium arsenide (GaAs), and indium phosphide (InP). The semiconductor memory devices are largely classified into volatile data storage devices and nonvolatile data storage devices.
The volatile data storage devices lose stored data when a power supply is interrupted. Examples of the volatile data storage devices include static random access memory (SRAM) devices, dynamic random access memory (DRAM) devices, and synchronous dynamic random access memory (SDRAM) devices. The nonvolatile data storage devices retain stored data even when power supply is interrupted. Examples of the nonvolatile data storage devices include read-only memory (ROM) devices, programmable read-only memory (PROM) devices, erasable programmable read-only memory (EPROM) devices, electrically erasable programmable read-only memory (EEPROM) devices, flash memory devices, phase-change random access memory (PRAM) devices, magnetic random access memory (MRAM) devices, resistive random access memory (RRAM) devices, and ferroelectric random access memory (FRAM) devices. The flash memory devices are typically classified into NOR flash memory devices and NAND flash memory devices.
SRAM devices typically operate faster than DRAM devices. Accordingly, SRAM is mainly used for a memory device of a central processing unit (CPU) such as a register, which requires a fast operating speed. Moreover, SRAM is used for an external cache, a digital signal processing circuit, and so forth. However, SRAM is typically not suitable for a main memory of a personal computer (PC) because it typically has a lower degree of integration than DRAM.